1. Field of the Invention
This invention relates to a liquid water sensor for the prevention of corrosion in a pipeline used to transport a carbon dioxide-containing gas, i.e., natural gas. This invention further relates to an electrical circuit to determine whether liquid water is present at the liquid water sensor or another conductive material has fouled the sensor.
2. Discussion of the Art
Natural gas is a naturally occurring mixture of condensible hydrocarbons, and hydrocarbon and non-hydrocarbon gases found in the porous geological formations beneath the earth's surface, often in association with petroleum. To obtain a marketable product, the raw natural gas flowing from gas or oil wells must be gathered to control areas, and processed to remove water vapor, inert or poisonous constituents and condensible hydrocarbons. Once processed, the gas is principally methane in admixture with small amounts of ethane, propane, butanes, pentanes, carbon dioxide and nitrogen. The processed gas can be easily transported from producing areas to the market through underground pipelines for use in heating residential and commercial facilities and/or as a feed stream for various chemical processes.
It is critical that some water vapor be removed from the natural gas before transporting by pipeline since, at the operating pressures of commercial pipelines, ice-like gas hydrate crystals form at temperatures well above 0.degree. C. The buildup of these ice-like crystals may restrict or stop the gas flow. Carbon dioxide and nitrogen are inert gases which are removed from raw natural gas to improve the heating value of the remaining gas stream, e.g. it is common practice to limit the inert gases to three percent of the total gas volume. Hydrogen sulfide is also removed from raw natural gas because it is extremely poisonous. It is also well known that, in the presence of water, hydrogen sulfide and carbon dioxide are corrosive.
Unpurified natural gas, as in gas gathering systems, is dehydrated by passing through a solvent for water. For example, water may be removed by absorption in a hygroscopic liquid such as diethylene glycol or triethylene glycol or by adsorption on an activated solid such as silica gel, activated alumina, or a molecular sieve.
Solvents are also used to remove the so-called acid gases, carbon dioxide and hydrogen sulfide. However, in a typical oil or gas field, gas is gathered in a network of pipelines and is dehydrated at some point before the gas is transported to a central processing station for acid gas removal. The dehydration is accomplished in field installations which require monitoring for effectiveness. Failure of the dehydration equipment (or improper operation of it) will allow liquid water to exist in the pipeline downstream from the dehydration equipment. Since the gas not uncommonly contains carbon dioxide and often hydrogen sulfide as well in the gas gathering system, the pipeline transporting the gas could corrode if a liquid water phase is allowed to exist. The corrosion products would include ferrous carbonate and ferrous sulfide.
Thus it is an object of this invention to provide a novel water sensor for the detection of liquid water in a natural gas pipeline used in transporting a carbon dioxide-containing gas.
It is another object of this invention to detect liquid water in the presence of other electrically conductive materials having a boiling point greater than the boiling point of water.
It is another object of this invention to provide an electrical circuit for automatically detecting the presence of an electrically conductive material in a pipeline and determining whether such electrically conductive material is liquid water or an electrically conductive material such as ferrous sulfide.
It is another object of this invention to provide a method of preventing corrosion in a pipeline utilized to transport a carbon dioxide-containing gas by monitoring for the presence of liquid water and adjusting the dew point of said gas to evaporate said liquid water.
Other objects and advantages of the instant invention will become apparent from the following description taken in connection with the accompanying drawings wherein it is set forth by way of illustration and example certain embodiments of this invention.